Joint part determination method and joint material manufacturing method

A joint part determination method, comprising:

• an incidence step of causing ultrasonic waves to be incident to a joint part from a side opposite to an insertion side of a probe in the joint part when a workpiece formed by stacking members is joined by a tool for friction stir welding which includes the probe which is inserted into the workpiece and a shoulder which supports the probe;
• an image acquisition step of obtaining an ultrasonic wave transmission image of the joint part by the ultrasonic waves;
• and
• a determination step of determining that the joint part is joined in a case where a width dimension of the joint part obtained by the ultrasonic wave transmission image is equal to or more than an outer diameter of the probe and no joint defect of a predetermined size or more is found within a range equal to or less than an outer diameter of the shoulder in the ultrasonic wave transmission image.